Integumentary System

The integumentary system of an organism is its protective outer covering. All organisms have an integument or covering that separates the organism from its environment and serves several other important functions. The integument of vertebrates (animals with a backbone) is called skin. Skin can vary widely, from the impenetrable shell of an armadillo to the amazingly smooth skin of a porpoise.

Every organism has some sort of covering that holds together its body organs and fluids and makes it separate from its environment. This outer covering protects it from foreign bodies and matter and sometimes allows it to communicate with the world outside itself. In both one-celled organisms and plants, the integument is the same as its cell membrane and any secretion or coating that it produces. More complex invertebrates (animals without a backbone) have an integument that consists of a single layer of cells called an ectoderm. Only in vertebrates is the integument a many-layered, complex organ system that serves many functions.

THE INTEGUMENTARY SYSTEM OF PLANTS

The integument, or outer covering, of plants does the same thing that skin does for animals—it protects plants from injury and prevents underlying tissue from drying out. Higher plants, or those that have seeds and a vascular system (an internal system of tubing that carries fluid), have a living epidermis usually one cell thick. It may be thin, like the covering on lettuce leaves, or thick and tough like that on pine needles. The epidermis has openings called stomates that regulate temperature and water loss. It can have coatings like the wax on an apple or sensitive hairs like those on a Venus's flytrap. Overall, most plant integuments are fairly fragile compared to animal coverings.

THE INTEGUMENTARY SYSTEM OF INVERTEBRATES

The integument of more complex invertebrates is usually a single layer of cells that secrete some type of cuticle. The cuticle oozes out of the epidermis and hardens, affording it some type of protection. In crustaceans like crabs and lobsters, this functions as an external skeleton and is very hard and tough. Insects have an outer covering made up of chitin fibers that is secreted by the epidermis. Chitin forms a type of flexible, natural plastic and acts as an outer skeleton to which muscles are attached. In insects, the cuticle is a living structure and can produce sensitive hairs as well as bristles, scales, claws, or wings, depending on the species. Insects grow by shedding the cuticle and growing a new, larger one. Mollusks, like clams and snails, also form a hard, external shell.

INTEGUMENTARY SYSTEMS IN VERTEBRATES

Only in vertebrates, however, is the integumentary system considered to be a vital organ. That is, it not only provides protection for the delicate tissue underneath, but it gathers and conveys information to the organism itself about the outside environment. The skin of all vertebrates consists of two layers: the relatively thin (outer) epidermis, and the tough, inner dermis. The epidermis is several cells thick, and its outermost layer of cells is made up of dead cells composed of keratin, the protein found in hair, nails, claws, beaks, feathers, scales, and quills, among other things. Vertebrates replace this outer layer of dead cells every twenty-eight days, with the new layer identical to the old. In mammals, the inner dermis is highly developed, and is richly supplied with blood vessels, glands, and nerve endings. Besides acting as a barrier against infection and retaining the body's fluids, the dermis also has a regulatory function of letting the organism know whether to raise or lower body temperature or to move to where it is cooler or warmer. In endothermic (warm-blooded) animals, the skin plays an important role in regulating the body's temperature. In humans, the epidermis also contains a dark pigment called melanin that protects the skin from the Sun's ultraviolet radiation. It is the amount of melanin an individual's body produces that accounts for what we describe as the many different colorings of human "races."

Mammals. In mammals, the presence of hair is a distinguishing characteristic. As an outgrowth from a mammal's skin, hair grows from a pit in the dermis called a hair follicle. This pit also has a small gland that secretes an oily substance that keeps the hair oiled. For most mammals, the hair's main function is to act as insulation against the cold. Hair also serves as a sensory organ (like long whiskers) for certain night-prowling animals. Eyelashes in humans serve to make the eyes reflexively shut if they are hit by a speck of dust. There are also other glands in the dermis that keep the skin oiled and waterproof. Humans also have sweat glands in the dermis that act as a temperature control by means of evaporation, and most mammals also have dermal glands that produce odors that are thought to be a form of sexual communication.

In humans, the skin is considered the largest organ of the body. It changes considerably over time, and as it ages it becomes less elastic and more wrinkled. As with all vertebrates, human skin provides both protection from and communication with its environment. The dermis is rich with nerve fibers that can respond rapidly to changing environmental conditions, reporting its findings to the brain, which makes the necessary adjustments.

Besides its practical functions as barrier and regulator, the human skin also possesses the exquisite and indescribable sense of touch. Its surface is usually home to many bacteria, mostly harmless, and the skin can be subject to many diseases or injuries.